The present invention relates generally to communication systems and in particular, to a method and apparatus for synchronization within such communication systems.
When a mobile unit is first powered on, there can exist a frequency offset between the mobile unit and the base station. This frequency offset is due to inaccuracies in the local frequency synthesizer (which are typically xc2x15 ppm). Because of this a mobile will have to perform a frequency acquisition stage by an automatic frequency control unit in order to synchronize to the base station. During frequency acquisition, a mobile unit typically estimates the frequency offset based on a phase offset between Walsh symbols. In particular, a frequency offset (xcex94ƒ) is usually determined as shown in FIG. 1 by determining a phase difference (xcex94xcex8) between two adjacent Walsh symbols 101-102 such that,             Δ      ⁢              xe2x80x83            ⁢      f        =                  Δ        ⁢                  xe2x80x83                ⁢        θ                    2        ⁢                  xe2x80x83                ⁢        π        ⁢                  xe2x80x83                ⁢        T              ,      xe2x80x83    ⁢            -      π         less than           Δ      ⁢              xe2x80x83            ⁢      θ         less than     π    ,
where T is the time interval between two adjacent Walsh symbols 101-102. It is worth noting that condition xe2x88x92xcfx80 less than xcex94xcex8 less than xcfx80 is necessary to avoid phase ambiguity. Consequently, for a given xcex94ƒ, the maximum allowed T is then       T     less than           π              2        ⁢        π        xc3x97                  "LeftBracketingBar"                      Δ            ⁢                          xe2x80x83                        ⁢            f                    "RightBracketingBar"                      =            1              2        ⁢                  "LeftBracketingBar"                      Δ            ⁢                          xe2x80x83                        ⁢            f                    "RightBracketingBar"                      .  
If the Walsh symbol time duration (T) is larger than xc2xd|xcex94ƒ|, frequency offset estimation will fail due to the phase ambiguity. Therefore, a need exists for method and apparatus for synchronization within a spread-spectrum communication system that reduces the chance of phase ambiguity.